1. Technical Field of the Invention
The invention relates to a delta robot, comprising a stationary base plate and at least three drives fastened thereon, which are in each case connected to at least one arm, of which at least one is connected at the other end thereof to two rods by way of a ball joint, said rods running parallel to each other and at the other end thereof being connected, via a further ball joint, to a movable parallel plate, wherein each ball joint has a ball segment head and a hollow ball segment designed complementary thereto, and the two ball joints, which adjoin each other on parallel rods, are oriented mirror-symmetrically to each other, and each hollow ball segment is pressed onto the associated ball segment head by tensioning at least one permanently elastic element between two rods that are parallel to each other.
2. Description of the Prior Art
In the prior art, delta robots have proven their worth in particular for packaging lightweight foods, since they permit extremely high dynamics of up to three packaging operations per second.
The first delta robots had three arms. However, variants with four and more arms are known.
In the prior art, most delta robots consist of a base plate on which three servo drives are mounted. At the drive shaft thereof, an arm—also known as upper arm—is attached, on which, as lower arm, two parallel rods are pivotably mount, which in turn are in pivotable connection with the parallel plate. By a swivelling of the servo drives, the parallel plate can be manoeuvred into any desired position of the available work space. Because the underarms consist of two parallel rods, the parallel plate thus always moves—as its name suggests—parallel to the base plate.
In principle, a universal joint or cardan joint can be used for the articulated connection of the rods to the arms and to the parallel plate. The maximum pivoting angles that can thereby be achieved, however, are significantly lower in comparison to a ball joint, consisting of a ball segment head and a complementary hollow ball segment sliding thereon. Because ball joints thus have a larger pivot angle and thereby permit a larger working space, they are now the joint design most used for delta robots.
In the prior art, the U.S. Pat. No. 5,333,514, Osamu Toyama, discloses a delta robot, the lower arm of which consists of parallel rod pairs, at the ends of which hollow ball segments, also designated ball sockets or ball cups, are arranged, and are pressed onto the ball segment heads on the arm or on the parallel plate in that the ends of the two parallel rod-shaped lower arms are held together by tensioning springs. The hollow ball segment then moves on the ball head in the manner of a sliding bearing. A considerable advantage of this bearing is that a similar contact area is produced at every angle.
The various depictions of the rods in U.S. Pat. No. 5,333,514 show the same width from all points of view, from which it follows that the rods have a round cross-section. In FIG. 5 of the patent, it can be seen that the rods are a thin-walled tube.
There, it is also shown that the opening surfaces of the hollow ball segments of the ball joints are oriented in an extension of the centre axis.
However, from this arrangement—which is typical in the prior art—it emerges as the major disadvantage that the rods are asymmetrically loaded, and therefore can easily bend, as is explained below: In a tube, the centre axis is also the longitudinal support axis. If a force acts in the direction of this longitudinal support axis, then the tube is precisely symmetrically loaded and the forces are distributed to all regions of the tube wall and the maximum of the load bearing capacity of the tube is achieved.
However, as soon as the loading migrates from the longitudinal support axle, part of the tube wall is more strongly loaded than the other parts, and collapses, although the maximum load-bearing strength of the tube has not yet been reached.
Since, in a ball joint, the forces are distributed beyond the contact surface, in the aforementioned arrangement, compresses forces that are introduced, via a ball head and the associated hollow ball segment, into a road are disadvantageously not transmitted into the longitudinal support axis, but outside thereof. Therefore, on that side at which the hollow ball segment is arranged, the tube will collapse already at compressive loads that lie below the maximum load-bearing capacity of the tube.